Main Linac Research Articles

Design of the RF waveguide network for the klystron-based CLIC main linac RF module

The klystron-based Compact Linear Collider (CLIC) was initially proposed with a low center-of-mass energy of 380 GeV, primarily due to potential cost-effectiveness. To enhance overall cost-efficiency, reliability, and stability, a novel RF module for klystron-based CLIC main linac has been designed and studied. This RF module utilizes two X-band klystrons to feed eight traveling wave accelerating structures, resulting in a beam energy increase of 138 MeV. A key innovation of the new RF module is the integration of double-height waveguides, which contributes to a 30% reduction in surface fields and a 40% decrease in RF loss. To meet the double-height requirement, a majority of the RF components responsible for transmitting the high RF power underwent a redesign. Two solutions, based on the choke mode flange and the L-shape waveguide, were proposed to facilitate the adjustment of the accelerating structures for beam-based alignment. Additionally, bent damping waveguides and HOM loads and a high order Magic-T, were designed for the accelerating structure named CLIC-K. The paper will comprehensively present and describe both the RF design and the integration of the klystron-based CLIC module.

Design, testing, and validating the CLIC module pre-alignment and alignment systems

The Compact Linear Collider requires micron-level alignment of the key accelerator components within the two 21 km main LINACs to achieve the desired luminosity. Several Super Accelerating Structures (SAS) will be pre-aligned on a single common support structure to within 14μm. This common support structure will then be actively aligned relative to a Machine Reference Network, and in operation beam-based alignment will be used to achieve the final 1 μm alignment required. A design for the SAS pre-alignment system has been created, prototyped, and tested, and shown to match the design performance within 2.3 μm/turn, and to meet the specification requirements. A design for the active girder alignment system has also been created, prototyped, and initial testing has begun. This testing indicates that the system works as designed. Finally, the testing required to confirm this, and to fully validate the CLIC alignment procedure, is discussed.

Status and future plans for C3 R&D

C3 is an opportunity to realize an e + e - collider for the study of the Higgs boson at √s = 250 GeV, with a well defined upgrade path to 550 GeV while staying on the same short facility footprint [2,3]. C3 is based on a fundamentally new approach to normal conducting linear accelerators that achieves both high gradient and high efficiency at relatively low cost. Given the advanced state of linear collider designs, the key system that requires technical maturation for C3 is the main linac. This paper presents the staged approach towards a facility to demonstrate C3 technology with both Direct (source and main linac) and Parallel (beam delivery, damping ring, ancillary component) R&D.The primary goal of the C3 Demonstration R&D Plan is to reduce technical and cost risk by building and operating the key components of C3 at an adequate scale. This R&D plan starts with the engineering design, and demonstration of one cryomodule and will culminate in the construction of a 3 cryomodule linac with pre-production prototypes. This R&D program would also demonstrate the linac rf fundamentals including achievable gradient and gradient stability over a full electron bunch train and breakdown rates. It will also investigate beam dynamics including energy spread, wakefields, and emittance growth. This work will be critical to confirm the suitability of the C3 beam parameters for the physics reach and detector performance in preparation for a Conceptual Design Report (CDR), as well as for follow-on technology development and industrialization.The C3 Demonstration R&D Plan will open up significant new scientific and technical opportunities based on development of high-gradient and high-efficiency accelerator technology. It will push this technology to operate both at the GeV scale and mature the technology to be reliable and provide high-brightness electron beams.The timeline for progressing with C3 technology development will be governed by practical limitations on both the technical progress and resource availability. It consists of four stages: Stage 0) Ongoing fundamental R&D on structure prototypes, damping and vibrations. Stage 1) Advancing the engineering maturity of the design and developing start-to-end simulations including space-charge and wakefield effects. This stage will include testing of strucutres operating at cryogenic temperatures. Beam tests would be performed with high beam current to test full beam loading. Stage 2) Production and testing of the first cryomodule at cryogenic temperatures. This would provide sufficient experimental data to compile a CDR and it is anticipated for Stage 2 to last 3 years and to culminate with the transport of photo-electrons through the first cryomodule. Stage 3) Updates to the engineering design of the cryomodules, production of the second and third cryomodule and their installation. Lower charge and lower emittance beams will be used to investigate emittance growth. The successful full demonstration of the 3 cryomodules to deliver up to a 3 GeV beam and achieve the C3five gradient will allow a comprehensive and robust evaluation of the technical design of C3 as well as mitigate technical, schedule, and cost risks required to proceed with a Technical Design Report (TDR).

Design Study of a Superconducting Quadrupole Magnet System Sustainable Under Dark Current Heating in ILC Main Linac

The International Linear Collider (ILC) main linac comprises a series of 12 m-long cryomodules. The cryomodule contains eight 9-cell superconducting (SC) RF (SRF) cavities and an SC quadrupole magnet combined with dipole correctors to focus and steer electron and positron beams. The magnets are installed between the SRF cavity string and at the longitudinal center of a common cryomodule/cryostat. These magnets are conductively cooled by pure aluminum channels thermally connected to a 2 K two-phase helium pipe for cooling the SRF cavities. A recent study shows that field-emitted electrons, so-called “dark current” initiated in the SRF cavities, are transmitted through the SRF cavity string and reach the SC magnet. The energy is inevitably absorbed in the SC coil due to the magnetic field, resulting in risks of a quench caused by the coil heating. We are investigating alternate magnet designs by using Nb <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula> Sn or <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{MgB}_{2}$</tex-math></inline-formula> SC and by adding the dark current absorber surrounding the beam-pipe to realize sustainable magnet operation under the dark current heating. We report the design study of the magnet system and interfaces to the cryomodule accommodating it with the SRF cavities.

Characterization and alignment of a resonant cavity beam position monitor for the Compact LInear Collider (CLIC) project at CERN

Beam diagnostic devices are essential tools to sense the properties of the beam in particle accelerators. Beam Position Monitors (BPMs) are used to detect the transverse beam spatial coordinates. A large number of high-resolution resonant cavity BPMs, operating at 14GHz, are proposed to monitor the orbit of the two main beam linacs of the CLIC project. This paper discusses innovative calibration techniques on a dedicated test bench in the frame of the PACMAN project. The particular location of the electromagnetic center of the resonant cavity BPM is detected utilizing a stretched-wire measurement approach. Measurements, RF characterization and final fiducialization of the BPM electrical offset are treated with details. Initial studies through EM simulations of the cavity BPM are covered. The presented experimental results prove the feasibility of the alignment methodology with nanometric resolution.

SwissFEL double bunch operation

SwissFEL has a unique capability, among the normal conducting linac-based light sources, to simultaneously serve two separate undulator lines (Aramis and Athos) up to the machine repetition rate of 100 Hz using the double bunch operation mode. It increases twice the experiments throughput of the facility with modest additional investment. Two electron bunches spaced 28 ns apart are extracted from the cathode by two laser pulses with individually controlled repetition rates. The bunches are accelerated up to about 3 GeV in the main linac using the same rf macropulse. After separation, one bunch serves the Athos soft x-ray beamline and the other is further accelerated to serve the hard x-ray beamline – Aramis. A fast and high-stability beam kicker separates the two bunches without disturbing the electron beam and consequently the x-ray lasing. The timing and control system sets hybrid machine modes utilizing independent operation of the two undulator lines with individually programmed repetition rates. Beam diagnostics and feedback systems have to operate with two closely spaced bunches where the two beams share the same machine path. The low-level rf system manipulates the rf amplitude and phase within a fraction of the rf macropulse to provide decoupling of the acceleration parameters of the first and the second bunch. This manuscript presents measurements that show that the bunch separation does not degrade FEL lasing stability.13 MoreReceived 7 July 2022Accepted 4 November 2022DOI:https://doi.org/10.1103/PhysRevAccelBeams.25.120701Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAccelerator/storage ring control systemsBeam diagnosticsBeam injection, extraction & transportBeam optics correction schemesBeam optics transportBeam processesBeam techniquesRadio frequency power sourcesAccelerators & Beams

Study on Conduction Cooling of Superconducting Magnets for the ILC Main Linac

In the main linac of the International Linear Collider (ILC), superconducting magnets for beam focusing and steering will be located periodically in superconducting RF (SRF) cavity string for beam acceleration in common cryomodules. A concept of conduction cooling of the combined-functioned, splittable superconducting magnets has been proposed and investigated to adapt much different features and to meet different requirements for the superconducting magnet and SRF cavity in fabrication, assembly, and operation. It is required to integrate the superconducting magnet after the SRF cavity string assembly which completed under an ultra-clean environment. The magnet must be conductively cooled down through thermal links to a liquid helium supply pipe. According to this concept, a model magnet development was carried out in cooperation with Fermilab and KEK, and has been demonstrated in KEK superconducting RF test facility (STF). In addition, an important issue has been recently identified. High gradient SRF cavities naturally emit field emission electron flux from the inner surface, so-called dark current. It may pass through the subsequent SRF cavity string and penetrate into the superconducting magnets placed downstream. It may heat up the superconducting coils, and may cause a quench. Therefore, further study on reliable conduction cooling and to secure the superconducting magnet operation with a keeping sufficient safety margin is quite essential. In this paper, we report the installation, the improvement achieved in STF, and the R&amp;D progress in the study on the conduction cooling of the superconducting magnet for the ILC main linac.

Design and optimisation of the Compact Linear Collider main LINAC module for micron-level stability and alignment

The Compact Linear Collider (CLIC) study is developing a Multi-TeV e+e− linear collider. An acceleration gradient of 100MVm−1 will be achieved in the Main LINACs using 11.994GHz Super Accelerating Structures (SAS). To achieve the required luminosity, the SAS will be prealigned within modules to less than 14µm and maintain their position to within 1.4µm when exposed to local sources of mechanical noise. A module design is presented and Finite Element Analysis (FEA) is used to optimise the harmonic frequencies of this module and thereby minimise the potential impact of unknown sources of vibration. A module with a fundamental frequency of 60Hz is presented. Historical ground noise data from the LHC at CERN is used to statistically quantify the magnitude of SAS misalignments unavoidably induced by local ground noise. The one-standard-deviation average vertical misalignment due to ground noise is less than 0.044µm above 0.1Hz for all SAS.

Novel open cavity design for rotating mode rf pulse compressors

A new $X$-band high-power rotating mode SLAC energy doubler (SLED)-type rf pulse compressor is proposed. It is based on a novel cavity type, a single open bowl-shaped energy storage cavity with high-quality factor and compact size, which is coupled to the waveguide using a compact rotating mode launcher. The novel cavity type is applied to the rf pulse compression system of the main linac rf module of the klystron-based option of the Compact Linear Collider. Quasispherical rotating modes of ${\mathrm{TE}}_{1,2,4}$ and ${\mathrm{TE}}_{1,2,13}$ are proposed for the correction cavity and storage cavity of the rf pulse compression system, respectively. The storage cavity working at ${\mathrm{TE}}_{1,2,13}$ has a quality factor above 240 000 and a diameter less than 33 cm. The design of the pulse compressor and in particular of the high-Q cavity will be presented in detail.

CLIC pre-alignment — status and remaining challenges

The Compact Linear Collider (CLIC) is a study of a 3 TeV linear electron–positron (e+e−) accelerator and is a successor candidate for CERN’s Large Hadron Collider. The CLIC luminosity target is 5.9 × 1034 cm−2 s−1, which causes unprecedented pre-alignment requirements of its main linear accelerator (main linac). Along the 50 km long tunnel, the main components of any 200 m long section have to be positioned within 10 µm from a straight reference line. The pre-alignment challenge has been studied at CERN since the 1990s, and the main technical challenges have been solved. This article summarizes the positioning strategy and presents it to an audience outside the particle accelerator community. The methods can be of interest especially in the field of large-scale metrology. The positioning strategy consists of several steps or subsystems. The development of a straight reference line over tens of kilometers allows absolute positioning of accelerator components, while a process called fiducialization defines component reference axes with regard to alignment targets. Emphasis is on a support pre-alignment network that acts as a link between the straight reference line and fiducialization. The subsystems and remaining challenges in their development are presented. The chosen strategy’s potential is demonstrated experimentally by building a short test setup.

Design, fabrication and relative bunch length measurement of S-band pre-buncher

A 10 MeV, 3 kW RF electron linear accelerator (linac) is in operation at EBC, Kharghar. A new horizontal linac rated for 10 MeV, 5 kW has been planned to be developed at EBC, Kharghar. The components of 10 MeV linac beam line consists of electron gun, a standing wave (SW) single cell pre-buncher cavity and a SW main linac cavity of length 0.9 m. In order to enhance the beam capture efficiency and to reduce the energy spread a pre-buncher cavity along with focusing elements will be introduced between the electron gun and the main linac cavity section. In this work the design simulation, fabrication, RF measurements and beam experimental trials of the 2856 MHz low Q, wide band pre-buncher cavity is presented. The measured RF parameters of pre-buncher cavity are f0=2854.4 MHz in air, QL=600, tuning range ±2.5 MHz. Electron beam transmission efficiency of 99% is achieved with minimum relative bunch length obtained at 400W of input RF power.

Measurement of the per cavity energy recovery efficiency in the single turn Cornell-Brookhaven ERL Test Accelerator configuration

Prior to establishing operation of the world's first multiturn superconducting energy recovery linac, the Cornell-BNL energy recovery test accelerator was configured for one-turn energy recovery. In this setup, direct measurement of the beam loading in each of the main linac cavities demonstrated high energy recovery efficiency. Specifically, a total one-turn power balance efficiency of 99.4%, with per cavity power balances ranging from 99.2--99.8% was measured. When accounting for small particle losses occurring in the path length adjustment sections of the return loop, the per cavity power balances correspond to per cavity single particle energy recovery efficiencies ranging from 99.8 to 100.5%. A maximum current of $70\text{ }\text{ }\ensuremath{\mu}\mathrm{A}$, limited by the an incomplete radiation shielding installation around the main beam stop at the time of these measurements, was energy recovered.

The physics design of HEPS Linac bunching system

The High Energy Photon Source is a fourth-generation synchrotron radiation source being built in China. It is comprised of a storage ring, a full energy booster, a 500-MeV Linac and three transport lines. The pulse charge at the exit of the Linac is required to be up to 7 nC. Both single bunch and three bunches in a beam pulse might be needed according to the studies on instabilities of the booster and the beam dynamic of the main Linac. A bunching system consisting of both sub-harmonic and fundamental frequency structures is designed to meet the requirements. To obtain high transmission efficiency and control the emittance growth, multi-objective genetic algorithm is introduced to optimize the electromagnetic parameters of these structures. In this paper, detailed optimization process and the final results are presented. The total transmission efficiency of the bunching system is 97%, and the transmission efficiency of the main bunch is 95%. The normalized RMS emittance of 8.5 nC included beam charge is smaller than 60 mm mrad. The design of the bunching system is presented, and both the pulse charge and the beam emittance well meet the design requirement.

Tolerance study of travelling-wave accelerating structure for the main linac of the klystron-based first stage of Compact Linear Collider

This work aims to analyse the tolerances given by two effects on the performance of the travelling-wave accelerating structure: the reflection along the structure and the change in the nominal accelerating voltage, which is essential for the projects with a large number of accelerating structures. The detuning and dephasing caused by systematic temperature shifts, systematic geometric errors and random geometric errors has been investigated in this paper. RF parameters of travelling-wave accelerating structure CLIC-K designed for klystron-based first stage of CLIC is used to defining the fabrication shape accuracy specification as well as the working temperature control for the high-gradient structurer if tuning is unfeasible.

Incorporation of a MESA linac module into bERLinPro

bERLinPro is an Energy Recovery Linac (ERL) project, currently being set up at the Helmholtz-Zentrum Berlin fur Materialien und Energie, Berlin, Germany. bERLinPro is designed as - and for - experiments in accelerator physics and as a test bed for novel ERL components. MESA is an ERL project under construction at the Johannes Gutenberg- Universitt, Mainz, Germany. MESA is designed as a user facility to perform experiments in dark matter physics and precision measurements of natural constants. Despite the diverse goals, the main linac, providing the larger part of the particles energy, is fairly compatible. It is planned to test and run the MESA linac module in bERLinPro, prior to its usage in MESA. The goals and benefits of this unique cooperation for both projects are outlined in this paper. The necessary adaptions in bERLinPro, including hardware aspects, the new optics, and the scope of performance are described.

Beam Orbit Correction in the CLIC Main Linac Using a Small Subset of Correctors

Beam Orbit Correction in the CLIC Main Linac Using a Small Subset of Correctors

CEPC Linac design

The Circular Electron–Positron Collider (CEPC) is a 100-km ring [Formula: see text] collider for a Higgs factory. The injector of CEPC is composed of Linac and Booster. The Linac is a normal conducting S-band Linac with a frequency of 2860 MHz; it provides electron and positron beams at an energy of up to 10 GeV with 100 Hz repetition frequency of 100 Hz. The Linac design and dynamic simulation results are discussed in detail in this paper, including electron bunching system, positron source, electron bypass transport line, damping ring and main Linacs.

Active suppression of microphonics detuning in high QL cavities

Operation of superconducting radio frequency (SRF) cavities with high loaded quality factors is becoming increasingly preferred for applications which involve low beam loading including energy recovery linacs (ERL). Vibration induced microphonics detuning poses a major operational bottleneck in these low bandwidth systems, adversely affecting field stability. Besides passive measures of mitigating the vibration sources, modern SRF cavities are also attached to fast tuners incorporating piezoelectric actuators. We demonstrate the narrow band active noise control algorithm for realizing active resonance control and propose a modification based on the least mean square approach to adaptively tune the control parameters and study its stability and performance. We discuss our experience of using passive mitigation techniques while commissioning the main linac cryomodule of the Cornell-BNL ERL test accelerator and report a net reduction in peak detuning by more than a factor of 2 in its unstiffened cavities. Finally, we demonstrate stable performance of our resonance control system with consistent reduction of peak microphonics detuning by almost a factor of 2 on multiple cavities.

Design, fabrication, and high-gradient testing of X -band choke-mode damped structures

Choke-mode accelerating structures are higher-order-mode (HOM) damped structures that are relatively simple to fabricate and have low surface magnetic fields. $C$-band choke-mode accelerating structures have been successfully applied in free electron lasers. However, studies on $X$-band choke-mode structures for the main linac of the Compact Linear Collider (CLIC) currently remain at the theoretical design stage, and the high-gradient performance of a choke remains unknown. In this study, we designed and fabricated five different single-cell choke-mode accelerating structures and subjected them to high-gradient tests to study related radio frequency (rf) breakdown characteristics. High electrical field and small choke gap caused serious breakdowns in the choke, which mainly limited high-gradient performance. The choke-mode accelerating structure reached $121\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ at a breakdown rate of $8.30\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ per pulse per meter and a flat top pulse width of 250 ns. A new quantity proposed to give the high-gradient performance limitation of choke-mode accelerating structures due to rf breakdown. The new quantity was obtained from the summary of the high-gradient experiments and can be used as reference for high-gradient choke-mode accelerating structure design.

Beam-Based Beamline Element Alignment for the Main Linac of the 380 GeV Stage of CLIC

Beam-Based Beamline Element Alignment for the Main Linac of the 380 GeV Stage of CLIC